The present invention relates to coating fibers with a plastic, and more particularly, coating glass fibers with a thermoplastic.
Attempts to coat glass fibers with thermoplastic, either by impregnating glass mats with a melt of plastic or by drawing continuous fiber bundles through a melt, have met with difficulty in getting complete wetout in final formed composite parts. The problem is that the melt viscosity is much higher for most thermoplastics than it is for most thermosetting plastics with which these processes work well.
One process that addresses this problem is the Wiggins-Teape (W-T) process such as disclosed in U.S. Pat. No. 3,716,449. It comprises comminution of the thermoplastic and then contacting it with the glass fibers. Then the mixture of glass fibers and ground plastic is suspended in a foam created from a slurry The water is then removed, the foam "broken", and the resulting structure is a fiber mat with the plastic particles loosely bound in the matrix. This process has the advantage of not needing to melt the plastic to create the mat preform structures. The disadvantages of this process include the complex slurry handling, contamination by foaming materials, and the movement and loss of particles from the mat on subsequent handling since they were never molten in the presence of the fibers. The W-T process also cannot use continuous fibers, and the structure of the mat is uniform through it thickness since the slurry containing plastic particles and chopped fibers is well mixed. Thus the W-T process cannot make a non-uniform mat or blanket of fibers and resin particles.
It is therefore an object of the present invention to have a process and apparatus for applying a thermoplastic to a fiber that has simplified handling without movement and loss of material or addition of forming materials, can be used with continuous fibers, and can make mats with non-uniform fiber lengths throughout their thickness.